by: Dr. Jessica Young
Skin cancer: two of the most dreaded words in the human language. We’ve all been there before, wondering why we didn’t put on more sunscreen when we went riding after we watch our skin turn to a pretty and painful shade of pink.
Skin tumors are quite common problem in our equine friends. Sarcoids, squamous cell carcinomas and melanomas are just three of the common offenders we see a lot in the horse population. Surgical removal, cryotherapy (freezing), and chemotherapeutic drugs have been common treatments for equine skin tumors in the past. Unfortunately, these treatments can sometimes be temporary solutions to permanent problems as tumors can and often come back.
Melanomas are a very common skin tumor found in older, gray horses. Cutaneous melanomas affect approximately 80% of gray horses older than 15-years of age. They often present as small, hard lumps under the skin and they are found most commonly on the underside of the tail, the anus, or the sheath in geldings and stallions. They can also be found on the head under the ear or near the parotid salivary gland around the throatlatch area. They arise from a disturbance in normal melanin (dark skin pigment) metabolism in the skin. The rate of growth and metastasis varies with each individual horse. These tumors are typically benign and slow growing most commonly, but metastasis to internal structures has occurred. The main concerns with melanomas that develop around the perianal region and sheath are interference with defecation and urination if the tumors become large enough to obstruct the anus or prevent normal extension of the penis.
Treatment for melanomas depends on the size of the tumor and the location. Small, single tumors can often be treated by surgical removal or freezing, but large tumors or multiple tumors that are very close in proximity to each other are more difficult to treat and often return after freezing or surgery.
A new treatment, cimetidine, has been used with increasing frequency and success in the last few years. Cimetidine is an oral antihistamine that is typically used for the treatment of gastric ulcers and stomach conditions, but clinical research has now shown that cimetidine has a therapeutic benefit in horses with melanomas as well. Cimetidine treats melanomas by modifying the portion of the immune system that allows melanomas to grow unchecked and stimulates the cell-mediated immune response necessary to fight the tumor cells.
Cimetidine provides the greatest benefit to horses that have melanomas that are actively increasing in size and number. Tumors that have remained quiet and not changed in a number of years will probably not respond to treatment. Cimetidine is given orally 2-3 times daily for 8-12 weeks, and if a response is seen, the dose is modified until the lowest dose that keeps the tumor under control is reached. If horses respond well enough to the medication, in some instances treatment may be stopped all together. In some horses there may be no obvious change in tumor size though progression of the disease may be stopped for months to years. A good response to treatment is typically a reduction in the size of the tumor by 50% and no further growth or the appearance of new tumors. For horses with very large tumors, surgery is often performed to reduce the size of the tumor and cimetidine is then used to prevent the tumor from growing further.
To date, research has not shown any toxic side effects from long-term administration of cimetidine, and it has proven itself to be a worthy option in treatment of equine melanomas.